[PATCH] ARM: Fix virtual kernel memory printing for sparsemem

[Catalin Marinas]

On Thu, 2010-03-25 at 15:10 +0000, Russell King - ARM Linux wrote:
> On Thu, Mar 25, 2010 at 02:53:24PM +0000, Catalin Marinas wrote:
> > Commit db9ef1a introduced information printing for the virtual kernel
> > memory map but page/end calculation using the pfn goes wrong and
> > page_count() generates a data abort or alignment fault (possibly because
> > it gets to an uninitialised page structure that looks like a compound
> > page).
> 
> While this looks fine, I'd like to see a lot of Tested-by's against
> this before it's merged - we've had similar code in show_mem()
> which has proven to be quite problematical to get right for all the
> various different combinations we have.
> 
> However, we also have the same method in show_mem() which we know
> works fine, so I'd also like to see the problem with using it in
> mem_init() fully analysed - rather than a "possibly because".

I can remove the "possibly" part :). The page_count() is given a page
with some random flags and it thinks it's a compound page. It than tries
to access page->first which isn't set, hence the error.

The original code assumes that for a given node the map is contiguous
and it calculates the page as (map + pfn). This is only true for
flatmem. With sparsemem the page calculation is a bit more complicated
but handled by pfn_to_page().

-- 
Catalin